Method for Producing Olefins from Synthesis Gas in a Reaction Column

ABSTRACT

Process for the synthesis of olefins from synthesis gas in the presence of at least one Fischer-Tropsch catalyst in a reaction column, wherein the synthesis gas is introduced into the reaction column below a zone A of the reaction column and the olefins formed are taken off below the inlet for the synthesis gas.

The present invention relates to a process for the synthesis of olefins from synthesis gas in the presence of at least one Fischer-Tropsch catalyst in a reaction column.

The preparation of hydrocarbons from synthesis gas, i.e. a mixture of carbon monoxide and hydrogen, has been intensively researched for decades. This type of reaction is usually referred to as Fischer-Tropsch synthesis. Catalysts which are usually used in this reaction usually comprise metals of group VIIIB of the Periodic Table, in particular Fe, Co, Ni and/or Ru, as catalytically active metals (e.g. v. d. Laan et al. Catal. Rev.-Sci. Eng., 41, 255 (1999)).

Despite the intensive research activities hitherto, there is a need to optimize Fischer-Tropsch processes further. It is known that the product compositions, whose component can range from methane through higher alkanes, higher alkenes, etc., to aliphatic alcohols can be altered as a function of the chosen reaction conditions, the catalysts, etc. In addition, the exothermic nature of the Fischer-Tropsch process makes handling and in particular control of the reaction difficult.

If hydrocarbons enriched with olefins, preferably with α-olefins, are to be prepared, the catalysts used are generally ones which comprise nickel, cobalt, iron or ruthenium, in particular iron, iron and cobalt, iron/cobalt spinel or cobalt/manganese spinel, and also copper-promoted cobalt catalysts.

GB 1 512 743, GB 1 553 361, GB 1 553 362 and GB 1 553 363 describe catalytic processes for the synthesis of unsaturated hydrocarbons from synthesis gas at from 250 to 350° C. and from 10 to 30 bar. The catalysts used here comprise

-   (a) one or more oxides of the “difficult-to-reduce” metal oxides of     group IVB of the Periodic Table or a lower oxide of a transition     metal of group V or VII of the Periodic Table; and -   (b) one or more metals of group VII of the Periodic Table.     These catalysts can further comprise an alkali metal (group 1A of     the Periodic Table), magnesium oxide or zinc oxide as promoters.

U.S. Pat. No. 4,199,523 discloses a Fischer-Tropsch catalyst which comprises at least 60% of iron. Furthermore, this catalyst can comprise promoters such as copper, silver or alkali metals and/or other additives such as zinc oxide, manganese oxide, cerium oxide, vanadium oxide and chromium oxide.

In U.S. Pat. No. 4,418,155, Chang et al describe a process for the conversion of synthesis gas into hydrocarbons enriched with linear α-olefins, by bringing the synthesis gas at from about 260 to 345° C. into contact with a catalyst comprising a ZSM-5 type zeolite on which metals such as iron, cobalt or ruthenium have been deposited.

Furthermore, U.S. Pat. No. 5,100,856 describes copper/potassium-promoted iron/zinc catalysts which display improved activity, selectivity and stability in the synthesis of α-olefins from carbon monoxide and hydrogen.

It is likewise known that the composition of the hydrocarbons formed in the Fischer-Tropsch process can be strongly influenced by the choice of the catalysts used, the types of reactor and the reaction conditions.

WO 02/092216 describes, for example, a Fischer-Tropsch process over a monolithic catalyst support in a reactor which is divided into a plurality of reaction chambers in which the chemical reaction and the physical separation of the products take place. The product streams which are discharged from the various chambers differ in terms of their composition. For example, gasoline, kerosene and diesel are discharged separately from the reactor in the present case.

Despite the improvements which have been achieved to date, there continues to be a need for improvement of the commercially operated Fischer-Tropsch plants for the synthesis of olefins having from 4 to 20 carbon atoms.

It is an object of the present invention to provide a process for the synthesis of olefins, in particular α-olefins, from synthesis gas.

The object of the present invention is achieved by a process for the synthesis of olefins from synthesis gas in the presence of at least one Fischer-Tropsch catalyst in a reaction column, wherein the synthesis gas is introduced into the reaction column below a zone A of the reaction column and the olefins are taken off below the point at which the synthesis gas is fed in.

The reaction column used in the process of the invention comprises at least one top zone, a zone A and a bottom zone. Top zone, zone A and bottom zone are arranged in the stated order from the top downward in the reaction column. The zone A comprises at least one reaction zone and a distillation zone. The synthesis gas is introduced below the zone A but above the bottom zone and the olefins are taken off below the point at which the synthesis gas is fed in.

The Fischer-Tropsch catalyst is localized in the reaction zone and the Fischer-Tropsch synthesis takes place there.

The fractional distillation of the products formed in the Fischer-Tropsch synthesis takes place in the distillation zone.

However, it can also be the case that the zone of the chemical reaction and the zone of the physical separation (fractional distillation) are not physically separate. In this case, a combination zone is present. The combination zone is thus a combined reaction and distillation zone.

In the process of the invention, the synthesis gas is introduced into the reaction column below the zone A. The synthesis gas then comes into contact with the Fischer-Tropsch catalyst and a first hydrocarbon mixture a is formed; unreacted synthesis gas and volatile components of the hydrocarbon mixture formed then ascend into the next reaction zone where a further Fischer-Tropsch reaction takes place and a hydrocarbon mixture b is formed; this process is repeated. On the other hand, the volatility of the hydrocarbons formed decreases with increasing chain length and they are then present in liquid form and flow downward into the reaction zone(s) located underneath; there, chain extension by means of synthesis gas present can again take place; this process, too, is repeated. This finally results in a hydrocarbon mixture which can be taken off below zone A. This hydrocarbon mixture has, depending on the synthesis gas used, the Fischer-Tropsch catalyst and the process parameters (e.g. geometry of the reaction column, temperature profile of the reaction column, pressure, etc.), a particular molar mass distribution and a particular mean molecular weight. This molar mass distribution is preferably narrower than those of conventional Fischer-Tropsch hydrocarbons.

The process of the invention thus makes it possible to set firstly the mean molecular weight of the hydrocarbon mixture formed and secondly its molecular weight distribution by means of the superposition of the Fischer-Tropsch process on the distillation process. Furthermore, a higher selectivity to the desired reaction products, i.e. to olefins, in particular α-olefins, is achieved.

In one embodiment of the zone A, reaction and distillation zones alternate.

In a further embodiment of the zone A, combination and distillation zones alternate.

In a further embodiment of the zone A, a single combination zone is present.

In a further embodiment, synthesis gas is introduced at one or more points within the zone A in addition to the introduction of synthesis gas below the zone A. In some cases, it can be advantageous to carry out the additional introduction(s) into the distillation zone(s). However, it is also possible to carry out the introduction(s) into combination zone(s).

In a further embodiment, water in liquid form is fed in above or within the zone A.

However, it can also be advantageous to feed in water vapor below or within the zone A.

In a further embodiment, the Fischer-Tropsch catalyst which is localized in the reaction or combination zone forms a fixed bed, a fluidized bed, a suspension or a bubble column, preferably a fixed bed or a bubble column.

These embodiments can be implemented in a manner known per se to those skilled in the art, by, for example, applying the Fischer-Tropsch catalyst onto trays having a particular residence time of the condensate, for example valve trays, bubble cap trays or related constructions such as tunnel trays or Thormann trays, or fixing it on them as a catalyst bed.

However, it is also possible to introduce the Fischer-Tropsch catalyst into the column in the form of packing elements such as Raschig rings, Pall rings, saddle bodies appropriately provided with catalyst. Furthermore, it is possible to use packings comprising Fischer-Tropsch catalyst or to use mesh bags filled with Fischer-Tropsch catalyst, known as bales or Texas teabags. The packings as such are usually made of sheet metal, expanded metal, wire meshes or knitted meshes which preferably have a cross-channel structure. In these cases, combination zones are generally formed.

Internals having a distillative separation action are used in the distillation zones of the zone A. This can be achieved, for example, by means of trays, for example valve trays, bubble cap trays or related constructions, e.g. tunnel trays or Thormann trays, or sieve trays. However, it is also possible to use packings which usually comprise sheet metal, expanded metal, wire meshes or knitted meshes and preferably have a cross-channel structure. Examples are the packings Sulzer MELAPAK, Sulzer BX, Montz B1 types or Montz A3 types. However, it is also possible to use disordered packing elements, e.g. Raschig rings, Pall rings, saddle bodies, etc.

Preference is given to using reaction columns in which the zone A has from 5 to 150 trays, preferably from 15 to 100 trays, for carrying out the process of the invention.

A distillation zone usually comprises from 1 to 30 trays, a reaction zone usually comprises one tray and a combination zone usually comprises from 1 to 5 trays. This applies particularly when the reaction and distillation zones or the combination and distillation zones alternate.

In a further embodiment, a combination zone comprises from 20 to 100 trays.

A similar situation applies to the theoretical plates in the case of other column internals, i.e. when packings, etc., are used.

In a further embodiment, a reaction zone which is provided with packings or with Fischer-Tropsch catalysts in the form of packing elements provided with catalyst or with active distillation packings or with mesh bags filled with Fischer-Tropsch catalyst comprises from 20 to 100 theoretical plates.

In a particular embodiment, the zone A comprises from one to three distillation zones each having from 10 to 100 trays.

In a further particular embodiment, the zone A comprises a combination zone.

In a further embodiment, low boilers can be taken off via the top zone of the reaction column. These low boilers generally comprise inert gases such as nitrogen which may be present in the synthesis gas and also any carbon dioxide formed, low-boiling paraffins, in particular methane, low-boiling olefins such as ethene, etc.

In a further embodiment, low boilers formed, which comprise, for example, any low-boiling paraffins formed, low-boiling olefins and/or water, are taken off from zone A via a side offtake. The liquid product taken off via the side offtake can consist of two phases. It is possible for a phase separation to be carried out and the organic phase to be recirculated to the column. In this way, water can be specifically removed from the reaction zone.

In a further embodiment of the reaction column, the hydrocarbon mixture formed is removed from the reaction column below the point at which the synthesis gas is fed in. This can be achieved via a side offtake. However, it is also possible to take off the hydrocarbon mixture formed via the bottom of the column.

In a further embodiment, part of the hydrocarbon mixture formed is taken off from zone A via a side offtake and the other part of the hydrocarbon mixture formed is taken off below the point at which the synthesis gas is fed in.

In a further embodiment, the reaction column used comprises a top zone, a zone A and a bottom zone.

In a further embodiment, the reaction column used comprises a top zone, a zone A and a bottom zone and also a distillation zone B which is localized between the zone A and the bottom zone. Internals having a distillative separation action can be installed or packings can be comprised in this distillation zone. The embodiments of the internals or packings are analogous to those of the distillation zones of the zone A.

In a further embodiment, the reaction column used comprises a top zone, a zone A and a bottom zone and also a distillation zone C which is localized between the top zone and the zone A. Internals having a distillative separation action can be installed or packings can be comprised in this distillation zone. The embodiments of the internals or packings are analogous to those of the distillation zones of the zone A.

In a further embodiment, the reaction column used comprises a top zone, a zone A and a bottom zone and also a distillation zone B which is localized between the zone A and the bottom zone and a distillation zone C which is localized between the top zone and the zone A. Internals having a distillative separation action or packings can be comprised in these distillation zones B and C. The embodiments of the internals or packings are analogous to those of the distillation zones of the zone A.

The synthesis gas used in the process of the invention can be produced by generally known processes (as described, for example, in Weissermel et al., Industrial Organic Chemistry, Wiley-VCH, Weinheim, 2003, 15-24), for example reaction of coal or methane with steam or by comproportionation of methane with carbon dioxide. It usually has a ratio of carbon monoxide to hydrogen of from 3:1 to 1:3. Preference is given to using a synthesis gas which has a mixing ratio of carbon monoxide to hydrogen of from 1:0.5 to 1:2.5.

As catalysts, use is made of those Fischer-Tropsch catalysts which preferentially catalyze the formation of olefins, in particular α-olefins. Possible catalysts here are, in particular, Fischer-Tropsch catalysts comprising iron, iron and cobalt, iron/cobalt spinel or cobalt/manganese spinel and also copper-promoted cobalt Fischer-Tropsch catalysts. In particular, the catalysts described in GB 1 512 743, GB 1 553 361, GB 1 553 362, GB 1 553 363, U.S. Pat. No. 4,199,523, U.S. Pat. No. 4,418,155, U.S. Pat. No. 5,100,856 are incorporated by reference into the present invention.

The process of the invention is usually carried out at from 150 to 350° C. The pressure here is from 1 to 60 bar, preferably from 10 to 50 bar.

The GHSV (gas hourly space velocity) is generally from 100 to 30 000 parts by volume of feed stream per part by volume of catalyst and hour (l/l·h).

The product obtained in the process of the invention, which is removed from the reaction column below the point at which the synthesis gas is fed in, is a mixture of a plurality of hydrocarbons. This mixture has a particular mean molar mass and a particular molecular weight distribution. This product preferably comprises at least 50% by weight of olefins, preferably α-olefins.

The olefins obtained generally have from 4 to 20 carbon atoms, preferably from 5 to 14.

In a particular embodiment, a product comprising at least 50% by weight of olefins having from 5 to 7 carbon atoms, of which in turn at least 50% by weight is made up of one or more α-olefins, in particular 1-pentene and 1-hexene, is obtained.

In a further particular embodiment, a product comprising at least 50% by weight of olefins having from 8 to 14 carbon atoms, of which in turn at least 50% by weight is made up of one or more α-olefins, is obtained. These products obtained by the process of the invention are novel.

In a further particular embodiment, a product comprising at least 50% by weight of olefins having from 15 to 20 carbon atoms, of which in turn at least 50% by weight is made up of one or more α-olefins, is obtained. These products obtained by the process of the invention are novel.

Furthermore, it can be advantageous to introduce an α-olefin or a mixture of α-olefins whose number of carbon atoms is at least 1 less than that of the olefin mainly formed which can be separated off below the zone A into the zone A of the reaction column during start-up of the process. 

1-14. (canceled)
 15. A process for the synthesis of olefins from synthesis gas in the presence of at least one Fischer-Tropsch catalyst in a reaction column, wherein the synthesis gas is introduced into the reaction column below a zone A of the reaction column and the olefins formed are taken off below the inlet for the synthesis gas, wherein the zone A comprises at least one combined reaction and distillation zone (combination zone).
 16. The process according to claim 15, wherein the Fischer-Tropsch catalyst is localized in the reaction zone.
 17. The process according to claim 15, wherein the Fischer-Tropsch catalyst is localized in the combination zone.
 18. The process according to claim 15, wherein the zone A comprises at least one combination zone and at least one physically separate distillation zone.
 19. The process according to claim 15, wherein the zone A comprises at least one physically separate reaction zone and distillation zone.
 20. The process according to claim 15, wherein the olefins are α-olefins.
 21. The process according to claim 15, wherein the olefins formed are taken off via the bottom zone of the reaction column.
 22. The process according to claim 15, wherein synthesis gas is introduced at one or more points within the zone A in addition to the introduction of synthesis gas below the zone A.
 23. The process according to claim 15, wherein the catalyst forms a fixed bed, a fluidized bed, a suspension or a bubble column in a reaction or combination zone.
 24. The process according to claim 15, wherein the Fischer-Tropsch catalyst comprises at least one metal of group VIIIB.
 25. The process according to claim 15, wherein an α-olefin or a mixture of α-olefins whose number of carbon atoms is at least 1 less than that of the olefin mainly formed which is separated off below the zone A is introduced into the zone A during start-up of the reaction.
 26. A mixture of olefins obtainable by the processes according to claim
 15. 